Polymer hydrogels and methods of preparation thereof

ABSTRACT

The invention relates to a method for the preparation of a polymer hydrogel, comprising cross-linking a precursor comprising a hydrophilic polymer optionally in combination with a second hydrophilic polymer, using a polycarboxylic acid as the cross-linking agent. The invention further concerns the polymer hydrogel obtainable by the method of the invention and the use thereof in a number of different applications.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/150,430, filed on Jan. 8, 2014, which is a continuation of U.S.application Ser. No. 12/703,286 (now U.S. Pat. No. 8,658,147), filedFeb. 10, 2010, which is a continuation of International Application No.PCT/EP2008/006582, which designated the United States and was filed onAug. 8, 2008, published in English, which is a continuation-in-part ofInternational n Application No. PCT/IT2007/000584, which designated theUnited States and was filed on Aug. 10, 2007, published in English. Theentire teachings of the above applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to polymer hydrogels and methods ofpreparation thereof.

Polymer hydrogels are cross-linked hydrophilic polymers which arecapable of absorbing high amounts of water. In particular, cross-linkedpolymer hydrogels capable of absorbing an amount of water in excess of10 times their dry weight are defined as “superabsorbent”. Some of thesematerials are even capable of absorbing over 1 litre of water per gramof dry polymer.

The cross-links or cross-linking knots, i.e. the physical or chemicalbonds between the macromolecular chains forming the polymer hydrogelnetwork, guarantee the structural integrity of the polymer-liquidsystem, on the one hand preventing the complete solubilisation of thepolymer, and on the other hand allowing the retention of the aqueousphase within the molecular mesh.

The superabsorbent polymer hydrogels which are currently available onthe market are characterised not only by their marked absorbentproperties, but also by their biocompatibility, which is probably due tothe high water content, and, above all, by the possibility of adjustingtheir absorption properties according to the external stimuli.Consequently, such polymer hydrogels may be used as intelligentmaterials, for example for the manufacture of sensors or actuators for anumber of industrial applications. Besides the usual applications asabsorbent cores in the field of personal hygiene absorbent products,there are more recent and innovative applications such as for example inthe biomedical field, for the development of controlled release drugformulations, artificial muscles, sensors, etc., and in agriculture andhorticulture, for example in devices for the controlled release of waterand nutrients in arid soils.

However, the superabsorbent polymer hydrogels currently available arealmost exclusively acrylic-based products, and hence not biodegradable.

Given the growing interest in environmental protection issues, overrecent years a vast amount of interest has been focussed on thedevelopment of superabsorbent materials based on biodegradable polymers,having properties which are similar to those of the traditionalsuperabsorbent polyacrylics.

Examples of biodegradable polymers used to obtain superabsorbent polymerhydrogels are starch and cellulose derivatives.

In 1990 Anbergen and Oppermann [1] proposed a method for the synthesisof a superabsorbent material made entirely from cellulose derivatives.In particular, they used hydroxyethylcellulose (HEC) and acarboxymethylcellulose sodium salt (CMCNa), chemically cross-linked in abasic solution with divinylsulphone. However, the absorption propertiesof such materials are not high compared to those of the acrylic-basedsuperabsorbent materials.

In 1996 Esposito and co-workers [2], studying the synthetic processproposed by Anbergen and Opperman, developed a method for increasing theabsorption properties of the gel, acting mainly on the physicalproperties of the material. The basic idea was the induction ofmicroporosity into the polymer structure, so as to promote absorptionand retention of water by capillarity. Said microporosity was inducedduring the drying step, which was carried out by phase inversion in anonsolvent for the polymer, and the absorption properties of thematerial thus obtained were markedly superior to those of the air-driedgel.

CMCNa may be chemically cross-linked with any reagent which isbifunctional with respect to cellulose. Besides the divinylsulphone usedin the synthetic process according to Anbergen and Opperman,epichlorohydrin, formaldehyde and various diepoxides have also been usedas cross-linking agents. However, such compounds are highly toxic intheir unreacted states [3]. Some carbodiimides are known amongst theunconventional cross-linking agents. Particularly, the use ofcarbodiimides in order to cross-link salified or non-salifiedcarboxymethylcellulose (CMC) was described in [4]. Carbodiimides inducethe formation of ester bonds between cellulose macromolecules withoutparticipating in the bonds themselves, instead giving rise to a ureaderivative having very low toxicity [5]. A superabsorbent polymerhydrogel obtained by cross-linking carboxymethylcellulose sodium saltand hydroxyethylcellulose with carbodiimide as the cross-linking agentis disclosed in the international patent application WO 2006/070337 [6].

However, the carbodiimide used as a cross-linking agent in WO2006/070337 has the disadvantage of being extremely expensive. Moreover,during the cross-linking reaction with CMCNa, this substance turns intoa slightly toxic urea derivative, which must be removed during thewashing step, thereby further increasing the costs and the complexity ofthe production process. These drawbacks are extremely unfavourable,particularly in connection with those applications which require largescale production of the polymer hydrogels and which, consequently,involve high costs both with respect to the purchase of the startingmaterials and with respect to the disposal of the toxic substances whichare produced during synthesis.

Furthermore, the formation of substances having a certain degree oftoxicity, although very low, is a key factor for ruling out thepossibility of using such polymers in biomedical and pharmaceuticalapplications.

SUMMARY OF THE INVENTION

The object of the present invention is to provide polymer hydrogelswhich overcome the above-mentioned disadvantages associated with the useof carbodiimide as a cross-linking agent.

These and other objects are achieved by the polymer hydrogels of theinvention and the method of preparation thereof as defined herein. Thepolymer hydrogels of the invention are based on the use of apolycarboxylic acid, such as citric acid, as the cross-linking agent,and in preferred embodiments, also include the use of a molecularspacer.

The invention relates, in part, to the discovery that the cross-linkingof soluble cellulose derivatives with citric acid(3-carboxy-3-hydroxy-1,5-pentanedioic acid; hereinafter designated “CA”)results in the formation of polymer hydrogels and superabsorbent polymerhydrogels. CA is naturally occurring, non-toxic and available on themarket at low cost. Although CA has been reported as a cross-linkingagent for polymers such as cellulose, hydroxypropylmethylcellulose andstarch, in textile and food applications [7-11], in these applicationsCA is used to cross-link and further stabilize insoluble fibers, toprovide a fabric with enhanced resiliency and mechanical properties.However, the use of CA to cross-link carboxymethylcellulose or othersoluble hydrophilic polymers for preparing polymer hydrogels andsuperabsorbent polymer hydrogels has not been previously disclosed.

The method of preparing a polymer hydrogel according to the presentinvention comprises the step of cross-linking an aqueous solutioncomprising a hydrophilic polymer with a polycarboxylic acid, optionallyin the presence of a compound which functions as a molecular spacer.

In one embodiment, the aqueous solution comprises two or morehydrophilic polymers, such as, for example, hydroxylated polymers. Forexample, the aqueous solution can comprise a first hydrophilic polymerand a second hydrophilic polymer, which can be present in the same ordifferent amounts on a weight basis. In one embodiment, the firsthydrophilic polymer is an ionic polymer and the second polymer is anonionic polymer.

In one preferred embodiment, the invention provides a method forpreparing a polymer hydrogel, comprising the steps of (a) providing anaqueous solution of C carboxymethylcellulose, hydroxyethylcellulose,citric acid and a molecular spacer; (b) heating the aqueous solution,thereby evaporating the water and cross-linking thecarboxymethylcellulose and hydroxyethylcellulose to form a polymerhydrogel material; (c) washing the polymer hydrogel material with wateror a polar organic solvent to form a washed polymer hydrogel; (d)immersing the washed polymer hydrogel in a cellulose nonsolvent, therebyproducing a dried polymer hydrogel.

In yet another embodiment, the present invention provides polymerhydrogels, such as superabsorbent polymer hydrogels, which can beprepared using the methods of the invention. Such polymer hydrogelscomprise at least one hydrophilic polymer cross-linked with apolycarboxylic acid. Further, the invention includes articles ofmanufacture which comprise such polymer hydrogels.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 illustrates the proposed mechanism of polymer cross-linking bycitric acid.

FIG. 2 is a graph of cumulative food intake as a function of time forrats administered a polymer hydrogel of the invention orally and ratsadministered vehicle only.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides polymer hydrogels, methods of preparingthe polymer hydrogels, methods of use of the polymer hydrogels andarticles of manufacture comprising the polymer hydrogels.

The method of preparing a polymer hydrogel of the present inventioncomprises the step of cross-linking an aqueous solution comprising ahydrophilic polymer with a polycarboxylic acid, thereby producing thepolymer hydrogel. In some embodiments, the aqueous solution comprisestwo or more hydrophilic polymers. For example, the aqueous solution cancomprise a first hydrophilic polymer and a second hydrophilic polymer,which can be present in the same or different amounts on a weight basis.In preferred embodiments, the first hydrophilic polymer is an ionicpolymer and the second polymer is a nonionic polymer.

The cross-linking reaction is preferably conducted at elevatedtemperature, for example, at a temperature greater than room temperature(25° C.). For example, the reaction can be conducted at a temperaturefrom about 30° C. to about 150° C., preferably from about 50° C. toabout 120° C. In one embodiment, while the cross-linking reaction isconducted at elevated temperature, the reaction solution is concentratedby removal of water. The removal of water can be accomplished, forexample, by evaporation. In one embodiment, a fraction of the water isremoved. In another embodiment, substantially all of the water isremoved, thereby producing a dry residue. Optionally, the reactionmixture is maintained at elevated temperature for a period of timefollowing removal of water to dryness.

As used herein, the term “hydrophilic polymer” refers to a polymer whichis substantially water-soluble and, preferably, includes monomeric unitswhich are hydroxylated. A hydrophilic polymer can be a homopolymer,which includes only one repeating monomeric unit, or a copolymer,comprising two or more different repeating monomeric units. In apreferred embodiment, the hydrophilic polymer is hydroxylated, such aspolyallyl alcohol, polyvinyl alcohol or a polysaccharide. Examples ofsuitable polysaccharides include substituted celluloses, substituteddextrans, starches and substituted starches, glycosaminoglycans,chitosan and alginates.

Polysaccharides which can be used include alkylcelluloses, such asC₁-C₆-alkylcelluloses, including methylcellulose, ethylcellulose andn-propylcellulose; substituted alkylcelluloses, includinghydroxy-C₁-C₆-alkylcelluloses andhydroxy-C₁-C₆-alkyl-C₁-C₆-alkylcelluloses, such ashydroxyethylcellulose, hydroxy-n-propylcellulose,hydroxy-n-butylcellulose, hydroxypropylmethylcellulose,ethylhydroxyethylcellulose and carboxymethylcellulose; starches, such ascorn starch, hydroxypropylstarch and carboxymethylstarch; substituteddextrans, such as dextran sulfate, dextran phosphate anddiethylaminodextran; glycosaminoglycans, including heparin, hyaluronan,chondroitin, chondroitin sulfate and heparan sulfate; and polyuronicacids, such as polyglucuronic acid, polymanuronic acid, polygalacturonicacid and polyarabinic acid.

As used herein, the term “ionic polymer” refers to a polymer comprisingmonomeric units having an acidic functional group, such as a carboxyl,sulfate, sulfonate, phosphate or phosphonate group, or a basicfunctional group, such as an amino, substituted amino or guanidyl group.When in aqueous solution at a suitable pH range, an ionic polymercomprising acidic functional groups will be a polyanion, and such apolymer is referred to herein as an “anionic polymer”. Likewise, inaqueous solution at a suitable pH range, an ionic polymer comprisingbasic functional groups will be a polycation. Such a polymer is referredto herein as a “cationic polymer”. As used herein, the terms ionicpolymer, anionic polymer and cationic polymer refer to hydrophilicpolymers in which the acidic or basic functional groups are not charged,as well as polymers in which some or all of the acidic or basicfunctional groups are charged, in combination with a suitablecounterion. Suitable anionic polymers include alginate, dextran sulfate,carboxymethylcellulose, hyaluronic acid, polyglucuronic acid,polymanuronic acid, polygalacturonic acid, polyarabinic acid;chrondroitin sulfate and dextran phosphate. Suitable cationic polymersinclude chitosan and dimethylaminodextran. A preferred ionic polymer iscarboxymethylcellulose, which can be used in the acid form, or as a saltwith a suitable cation, such as sodium or potassium.

The term “nonionic polymer”, as used herein, refers to a hydrophilicpolymer which does not comprise monomeric units having ionizablefunctional groups, such as acidic or basic groups. Such a polymer willbe uncharged in aqueous solution. Examples of suitable nonionic polymersfor use in the present method are polyallylalcohol, polyvinylalcohol,starches, such as corn starch and hydroxypropylstarch, alkylcelluloses,such as C₁-C₆-alkylcelluloses, including methylcellulose, ethylcelluloseand n-propylcellulose; substituted alkylcelluloses, includinghydroxy-C₁-C₆-alkylcelluloses andhydroxy-C₁-C₆-alkyl-C₁-C₆-alkylcelluloses, such ashydroxyethylcellulose, hydroxy-n-propylcellulose,hydroxy-n-butylcellulose, hydroxypropylmethylcellulose, andethylhydroxyethylcellulose.

As used herein, the term “polycarboxylic acid” refers to an organic acidhaving two or more carboxylic acid functional groups, such asdicarboxylic acids, tricarboxylic acids and tetracarboxylic acids, andalso includes the anhydride forms of such organic acids. Dicarboxylicacids include oxalic acid, malonic acid, maleic acid, malic acid,succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid,azelaic acid, sebacic acid, phthalic acid, o-phthalic acid, isophthalicacid, m-phthalic acid, and terephthalic acid. Preferred dicarboxylicacids include C₄-C₁₂-dicarboxylic acids. Suitable tricarboxylic acidsinclude citric acid, isocitric acid, aconitic acid, andpropane-1,2,3-tricarboxylic acid. Suitable tetracarboxylic acids includepyromellitic acid, 2,3,3′,4′-biphenyltetracarboxylic acid,3,3′,4,4′-tetracarboxydiphenylether,2,3′,3,4′-tetracarboxydiphenylether,3,3′,4,4′-benzophenonetetracarboxylic acid,2,3,6,7-tetracarboxynaphthalene, 1,4,5,7-tetracarboxynaphthalene,1,4,5,6-tetracarboxynaphthalene, 3,3′,4,4′-tetracarboxydiphenylmethane,2,2-bis(3,4-dicarboxyphenyl)propane, butanetetracarboxylic acid, andcyclopentanetetracarboxylic acid. A particularly preferredpolycarboxylic acid is citric acid.

The method can further include the steps of purifying the polymerhydrogel, for example, by washing the polymer hydrogel in a polarsolvent, such as water, a polar organic solvent, for example, analcohol, such as methanol or ethanol, or a combination thereof. Thepolymer hydrogel immersed in the polar solvent swells and releases anycomponent, such as by-products or unreacted polycarboxylic acid that wasnot incorporated into the polymer network. Water is preferred as thepolar solvent, distilled water is still more preferred. The volume ofwater required during this step to reach the maximum swelling degree ofthe gel, is approximately 10- to 20-fold greater than the initial volumeof the gel itself. Taking into account the substantial amounts of waterwhich would be involved during this step on an industrial scale, as wellas the disposal and/or recycling of the washes, the importance ofavoiding the presence of any toxic by-products in the synthetic processbecomes evident. The polymer hydrogel washing step may be repeated morethan once, optionally changing the polar solvent employed. For example,the polymer hydrogel can be washed with methanol or ethanol followed bydistilled water, with these two steps optionally repeated one or moretimes.

The method can further include drying of the polymer hydrogel. Thedrying step is carried out by immersing the fully swollen polymerhydrogel in a cellulose nonsolvent, a process known as phase inversion.Suitable cellulose nonsolvents include, for example, acetone andethanol. Drying the polymer hydrogel by phase inversion results in afinal microporous structure which improves the absorption properties ofthe polymer hydrogel by capillarity. Moreover, if the porosity isinterconnected or open, i.e. the micropores communicate with oneanother, the absorption/desorption kinetics of the gel will be improvedas well. When a completely or partially swollen gel is immersed into anonsolvent, the gel undergoes phase inversion with the expulsion ofwater, until the gel precipitates in the form of a vitreous solid aswhite coloured particles. Various rinses in the nonsolvent may benecessary in order to obtain the dried gel in a short period of time.For example, when the swollen polymer hydrogel is immersed in acetone asthe non-solvent, a water/acetone mixture is formed which increases inwater content as the polymer hydrogel dries; at a certain acetone/waterconcentration, for example, about 55% in acetone, water is no longerable to exit from the polymer hydrogel, and thus fresh acetone has to beadded to the polymer hydrogel to proceed with the drying process. Thehigher the acetone/water ratio during drying, the faster is the dryingprocess. Pore dimensions are affected by the rate of the drying processand the initial dimensions of the polymer hydrogel particles:largerparticles and a faster process tend to increase the pore dimensions;pore dimensions in the microscale range are preferred, as pores in thissize range exhibit a strong capillary effect, resulting in the highersorption and water retention capacity.

The polymer hydrogels of the invention can also be dried by anotherprocess, such as air drying, freeze drying or oven drying. These dryingmethods can be used alone, in combination, or in combination with thenon-solvent drying step described above. For example, the polymerhydrogel can be dried in a non-solvent, followed by air drying, freezedrying, oven drying, o a combination thereof to eliminate any residualtraces of nonsolvent. Oven drying can be carried out at a temperature ofe.g. approximately 30-45° C. until the residual nonsolvent is completelyremoved. The washed and dried polymer hydrogel can then be used as is,or can be milled to produce polymer hydrogel particles of a desiredsize.

The cross-linking solution can optionally include a compound whichserves as a molecular spacer. A “molecular spacer”, as this term is usedherein, is a polyhydroxylated compound which, although not taking partin the reaction resulting in the formation of the cross-linked polymerhydrogel network to a significant extent, results in a polymer hydrogelwith an increased absorption capacity. Although in certain cases themolecular spacer may participate in the cross-linking reaction to asmall extent, it is believed that molecular spacers function bysterically blocking access to the polymer chains, thereby increasing theaverage distance between the polymer chains. Cross-linking, therefore,can occur at sites which are not close together, thereby enhancing theability of the polymer network to expand so as to greatly increase thepolymer hydrogel absorption properties. Suitable compounds for use asmolecular spacers in the methods of the present invention includemonosaccharides, disaccharides and sugar alcohols, including sucrose,sorbitol, plant glycerol, mannitol, trehalose, lactose, maltose,erythritol, xylitol, lactitol, maltitol, arabitol, glycerol, isomalt andcellobiose. The molecular spacer is preferably included in thecross-linking solution in the amount of about 0.5% to about 10% byweight relative to the solvent, more preferably about 2% to about 8% andmore preferably about 4%.

According to a preferred embodiment of the invention, the molecularspacer used to synthesise the polymer hydrogel is selected from thegroup consisting of sorbitol, sucrose and plant glycerol.

According to a particularly preferred embodiment of the method of theinvention, sorbitol is used as the molecular spacer, at a concentrationwithin the range of 0.5 to 10% by weight referred to the weight ofwater, preferably within the range of 2 to 8% by weight referred to theweight of water, still more preferably at a concentration of 4% byweight referred to the weight of water.

In one embodiment, the aqueous solution includes an ionic polymer,preferably an anionic polymer, and most preferably,carboxymethylcellulose. In a particularly preferred embodiment theanionic polymer is carboxymethylcellulose and the polycarboxylic acid iscitric acid.

In another embodiment, the aqueous solution includes an ionic polymerand a non-ionic polymer. The ionic polymer is preferably an anionicpolymer, and most preferably, carboxymethylcellulose. The non-ionicpolymer is preferably a substituted cellulose, more preferably ahydroxyalkylcellulose or a hydroxyalkyl alkylcellulose, and mostpreferably hydroxyethylcellulose (“HEC”). The preferred polycarboxylicacid is citric acid.

The weight ratios of the ionic and non-ionic polymers (ionic:non-ionic)can range from about 1:10 to about 10:1, preferably from about 1:5 toabout 5:1. In preferred embodiments, the weight ratio is greater than1:1, for example, from about 2 to about 5. In a particularly preferredembodiment, the ionic polymer is carboxymethycellulose, the non-ionicpolymer is hydroxyethylcellulose, and the weight ratio (ionic:nonionic)is about 3:1.

In a preferred embodiment of the method of the invention, which resultsin the formation of superabsorbent polymer hydrogels having aparticularly high swelling ratio (SR), the total precursor concentrationin the aqueous solution is of at least 2% by weight referred to theweight of the water of the starting aqueous solution, and the amount ofthe cross-linking agent is between about 1% and about 5% by weightreferred to the weight of the precursor. In the present description, theterm “precursor” indicates the hydrophilic polymer(s) used as theprecursors for the formation of the polymer hydrogel polymer network,for example, in certain embodiments the “weight of the precursor” is theweight of CMCNa used or the combined weights of CMCNa and HEC used. Theaqueous solution preferably includes sorbitol in an amount of about 4%by weight relative to the weight of water.

The swelling ratio (SR) is a measure of the ability of the polymerhydrogel to absorb water. SR is obtained through swelling measurementsat the equilibrium (using, for example, a Sartorius micro scale with asensitivity of 10⁻⁵) and it is calculated with the following formula:

SR=(W _(s) −W _(d))/W _(d)

wherein W_(s) is the weight of the polymer hydrogel after immersion indistilled water for 24 hours, and W_(d) is the weight of the polymerhydrogel before immersion, the polymer hydrogel having been previouslydried in order to remove any residual water.

According to the preparation method of the invention, in thisembodiment, the cross-linking reaction is preferably carried out at atemperature between about 60° C. and 120° C. Varying the temperatureduring this stage of the process will enable one to increase or decreasethe cross-linking degree of the polymer network. A cross-linkingtemperature of about 80° C. is preferred.

One particularly preferred embodiment of the method of the inventioncomprises the following steps: Step 1, the hydrophilic polymer(s), thecarboxylic acid and, optionally, the molecular spacer are dissolved inwater at room temperature; Step 2, the water is removed from thesolution at 40° C. over a two-day period; Step 3, the product of Step 2is heated to 80° C. for 10 hours to induce the cross-linking reactionand form a polymer hydrogel; Step 4, the polymer hydrogel is washedthree times with water over 24 hours; Step 5, the washed polymerhydrogel is immersed in acetone for 24 hours to remove water; Step 6,the polymer hydrogel is further dried in an oven at 45° C. for 5 hours;and Step 7, the dried polymer hydrogel is milled to provide polymerhydrogel particles.

The present invention also provides polymer hydrogels which can beprepared using the methods of the invention. Such polymer hydrogelscomprise a hydrophilic polymer cross-linked with a polycarboxylic acid.In other embodiments, the polymer hydrogels of the invention include atleast two hydrophilic polymers cross-linked by a polycarboxylic acid. Inone preferred embodiment, the polymer hydrogel comprises an ionicpolymer and a non-ionic polymer and a polycarboxylic acid, preferably aC₄ to C₁₂-dicarboxylic acid, a tricarboxylic acid or a tetracarboxylicacid, where the polycarboxylic acid cross-links the ionic polymer andthe non-ionic polymer. The weight ratio of ionic polymer to non-ionicpolymer is preferably from about 1:5 to about 5:1, more preferably fromabout 2:1 to about 5:1, and most preferably about 3:1. In oneparticularly preferred embodiment, the ionic polymer iscarboxymethylcellulose, the non-ionic polymer is hydroxyethylcelluloseand the polycarboxylic acid is citric acid. In another preferredembodiment, the polymer hydrogel comprises an ionic polymer, forexample, an anionic polymer or a cationic polymer. More preferably, theionic polymer is carboxymethylcellulose or a salt thereof, such assodium carboxymethylcellulose. In another particularly preferredembodiment, the polymer hydrogel comprises carboxymethylcellulosecross-linked with citric acid.

The polymer hydrogels of the invention have swelling ratios of at leastabout 5. Preferably, the polymer hydrogels of the invention aresuperabsorbent polymer hydrogels, for example, polymer hydrogels havingan SR of at least 10. In preferred embodiments, the polymer hydrogels ofthe invention have SRs at least about 20, about 30, about 40, about 50,about 60, about 70, about 80, about 90 or about 100. For example, incertain embodiments, the polymer hydrogels of the invention have SRsfrom about 10 to about 100, from about 20 to about 100, from about 30 toabout 100, from about 40 to about 100, from about 50 to about 100, fromabout 60 to about 100, from about 70 to about 100, from about 80 toabout 100, or from about 90 to about 100. In certain embodiments, theinvention includes polymer hydrogels having SRs up to 150, 200, 250,300, 330 or 350.

In certain embodiments, the polymer hydrogels of the invention canabsorb an amount of one or more bodily fluids, such as blood, bloodplasma, urine, intestinal fluid or gastric fluid, which is at least 10,20, 30, 40, 50, 60, 70, 80, 90, or 100 times their dry weight. Theability of the polymer hydrogel to absorb bodily fluids can be testedusing conventional means, including testing with samples of bodilyfluids obtained from one or more subjects or with simulated bodilyfluids, such as simulated urine or gastric fluid. In certain preferredembodiments, the polymer hydrogels can absorb significant amounts of afluid prepared by combining one volume of simulated gastric fluid (SGF)with eight volumes of water. SGF can be prepared using USP TestSolutions procedures which are known in the art. In some embodiments,the polymer hydrogels of the invention can absorb at least about 10, 20,30, 40, 50, 60, 70, 80, 90, 100 or more times their dry weight of thisSGF/water mixture.

The polymer hydrogels of the invention include cross-linked polymershaving varying extents of hydration. For example, the polymer hydrogelscan be provided in a state of hydration ranging from a substantially dryor anhydrous state, such as a state in which from about 0% to about 5%of the polymer hydrogel by weight is water or an aqueous fluid, tostates comprising a substantial amount of water or aqueous fluid,including up to a state in which the polymer hydrogel has absorbed amaximum amount of water or an aqueous fluid.

The polymer hydrogels of the invention can be used in methods fortreating obesity, reducing food or calorie intake or achieving ormaintaining satiety. The methods comprise the step of administering aneffective amount of a polymer hydrogel of the invention to the stomachof a subject, preferably by causing the subject, such as a mammal,including a human, to ingest the polymer hydrogel. Such polymerhydrogels can be used to take up stomach volume, for example, byincreasing the volume of a food bolus without adding to the caloriecontent of the food. The polymer hydrogel can be ingested by the subjectprior to eating or in combination with food, for example, as a mixtureof the polymer hydrogel with food. Upon ingestion and contact withgastric fluid or a combination of gastric fluid and water, the polymerhydrogel will swell. The polymer hydrogel can be ingested alone or in amixture with liquid or dry food in a dry, partially swollen or fullyswollen state, but is preferably ingested in a state of hydration whichis significantly below its fluid capacity, more preferably the polymerhydrogel is ingested in an anhydrous state. Thus, the volume of thestomach taken up by the polymer hydrogel can be significantly greaterthan the volume of the polymer hydrogel ingested by the subject. Thepolymer hydrogels of the invention can also take up volume and/or exertpressure on the wall of the small intestine by moving from the stomachinto the small intestine and swelling. Preferably, the polymer hydrogelwill remain swollen in the small intestine for a period of timesufficient to inhibit the intake of food by the subject, beforeshrinking sufficiently for excretion from the body. The time sufficientto inhibit the intake of food by the subject will generally be the timerequired for the subject to eat and for the ingested food to passthrough the small intestine, Such shrinking can occur, for example, bydegradation through loss of cross-links, releasing fluid and decreasingin volume sufficiently for excretion from the body. Preferred polymersfor use in this method exhibit pH-dependent swelling, with greaterswelling observed at higher pH than at lower pH. Thus, such a polymerwill not swell significantly in the stomach unless food and/or water ispresent to raise the pH of the stomach contents and will move into thesmall intestine. When ingested with food, the polymer hydrogel willinitially swell in the stomach, then shrink when the stomach is emptiedof food and the pH drops and then move from the stomach to the smallintestine. In the higher pH environment of the small intestine thepolymer hydrogel will swell, taking up volume in the small intestineand/or exerting pressure on the wall of the small intestine.

The polymer hydrogel can optionally be administered in combination witha pH modifying agent, which is an agent which alters the pH of themicroenvironment of the polymer hydrogel, thereby modifying its abilityto absorb fluids. For example, for polymer hydrogels comprising ananionic polymer, agents which increase the pH of the microenvironmentcan increase the swellability of the polymer hydrogel. Suitable pHmodifying agents for use with the polymer hydrogels of the inventioninclude buffering agents, H₂ blockers, proton pump inhibitors,anatacids, proteins, nutritional shakes, and combinations thereof.Suitable buffering agents and antacids include ammonium bicarbonate,sodium bicarbonate, calcium carbonate, calcium hydroxide, aluminiumhydroxide, aluminium carbonate, magnesium carbonate, magnesiumhydroxide, potassium bicarbonate, potassium carbonate, potassiumhydroxide, sodium carbonate, sodium hydroxide and combinations thereof.Suitable H₂ blockers include cimetidine, ranitidine, famotidine,nizatidine and combinations thereof. Suitable proton pump inhibitorsinclude omeprazole, lansoprazole, esorneprazole, pantoprazole,abeprazole, and combinations thereof.

The present polymer hydrogels can also be used for removing water fromthe gastrointestinal tract, for example, as a treatment for subjectssuffering from kidney disease, including chronic and acute kidneydisease, particularly subjects undergoing kidney dialysis. The polymerhydrogels can further be used to modify the fluid content in thegastrointestinal tract of a subject in need thereof, for example, forthe treatment of constipation.

The invention further includes articles of manufacture which comprisethe polymer hydrogels of the invention. Such articles of manufactureinclude articles in which polyacrylic polymer hydrogels areconventionally used, in consumer products, such as for example absorbentproducts for personal care (i.e., babies' napkins, sanitary towels,etc.) and in products for agriculture (e.g., devices for the controlledrelease of water and nutrients). The absorption properties of thepolymer hydrogels of the invention, which in some embodiments depend onthe amount of carboxymethylcellulose employed and which can be improvedby the induction of a microporosity in the gel structure, are comparableto those of polyacrylic gels. The polymer hydrogels obtainable by themethod of the present invention therefore possess mechanical propertieswhich make them suitable for use in all of the above-mentioned fields.The present polymer hydrogels, however, have advantages over acrylicpolymer hydrogels, such as biodegradability, the absence of any toxicby-products during the manufacturing process and the use of fewer andreadily available reagents. Such features enable a real employment ofthe polymer hydrogels of the invention in the biomedical andpharmaceutical fields as well.

Thus, the scope of the present invention also includes the use of thepolymer hydrogels obtainable by the method of the invention as anabsorbent material in products which are capable of absorbing waterand/or aqueous solutions and/or which are capable of swelling whenbrought into contact with water and/or an aqueous solution.

The polymer hydrogels and superabsorbent polymer hydrogels of thepresent invention may be used as absorbent materials in the followingfields, which are provided by way of non-limiting example:

dietary supplements (for example, as the bulking agents in dietarysupplements for hypocaloric diets capable of conferring a sensation oflasting satiety being retained into the stomach for a limited period oftime, or as water and low molecular weight compounds supplements, suchas mineral salts or vitamins, to be included into drinks in a dry orswollen form);

in agricultural products (for example, in devices for the controlledrelease of water and/or nutrients and/or phytochemicals, particularlyfor cultivation in arid, deserted areas and in all cases where it is notpossible to carry out frequent irrigation; such products, mixed in a dryform with the soil in the areas surrounding the plant roots, absorbwater during irrigation and are capable of retaining it, releasing itslowly in certain cases, together with the nutrients and phytochemicalsuseful for cultivation);

in personal hygiene and household absorbent products (such as forexample, as the absorbent cores in babies' napkins, sanitary towels andthe like);

in the field of toys and gadgets (such as for example in products whichare capable of significantly changing their size once brought intocontact with water or an aqueous solution);

in the biomedical field (for example, in biomedical and/or medicaldevices such as absorbent dressings for the treatment of highlyexudative wounds, such as ulcers and/or burns, or in slow-releasepolymeric films suitable to slowly release liquids adapted for use inophthalmology); and

in the body fluid management field, i.e., for controlling the amount ofliquids into the organism, for example in products capable of promotingthe elimination of fluids from the body, such as, for example, in thecase of edema, CHF (chronic heart failure), dialysis.

The above-mentioned products, containing a polymer hydrogel of thepresent invention as the absorbent material, also fall within the scopeof the invention.

The invention further includes the use of any of the polymer hydrogelsof the invention in medicine. Such use includes the use of a polymerhydrogel in the preparation of a medicament for the treatment of obesityor any medical disorder or disease in which calorie restriction has atherapeutic, palliative or prophylactic benefit.

The following examples are provided to further illustrate the inventionand are not to be construed as limiting its scope.

EXAMPLES

The materials and processes of the present invention will be betterunderstood in connection with the following examples, which are intendedas an illustration only and not limiting of the scope of the invention.Various changes and modifications to the disclosed embodiments will beapparent to those skilled in the art and such changes and modificationsincluding, without limitation, those relating to the chemicalstructures, derivatives, formulations and/or methods of the inventionmay be made without departing from the spirit of the invention and thescope of the appended claims.

Example 1 Citric Acid Cross-Linking ofCarboxymethylcellulose/Hydroxyethylcellulose Mixtures Materials

CMCNa (MW 700 kDa, DS 0.9, food grade), HEC (MW 250 kDa, food grade)were purchased from Eigenmann e Veronelli S.p.A. Milano and citric acidwas supplied by Dal Cin S.p.A. Sesto San Giovanni Milano and used asreceived.

Polymer Hydrogel Synthesis

Polymer hydrogel samples were obtained by reacting, in water, CMCNa andHEC with citric acid as a cross-linking agent according the followingprocedure. First, a total polymer concentration of 2% by weight ofwater, using a mixture of CMCNa and HEC, with weight ratio equal to 3/1was dissolved in distilled water by stirring gently at room temperatureuntil a clear solution was obtained. Poor cross-linking efficiency hasbeen reported if only CMCNa is used, due both to the electrostaticrepulsion between polyelectrolyte chains and to the high degree ofsubstitution of hydroxyl groups at C6, the most reactive position [13].CMCNa dissolution is slow at the concentration adopted; thus, first HECwas added to water till, after 5 min, a clear solution was obtained witha slight increase of viscosity; then, CMCNa was added, and the stirringwas kept on till a clear solution was obtained (24 h), with asignificant increase of viscosity. Finally, CA was added at differentconcentrations (1.75%, 2.75%, 3.75%, 10% and 20% w/w polymer) in orderto obtain samples with various degrees of cross-linking. This finalsolution was used to mold 10 mm thick samples. All samples were firstpre-dried at 30° C. for 24 h to remove absorbed water and then kept at80° C. for the cross-linking reaction (24 h with intermediate control).

Moreover, samples containing neat HEC or neat CMCNa samples cross-linkedwith CA were also prepared following exactly the same experimentalconditions used for HEC/CMCNa mixtures.

All samples were analyzed by FT IR measurements. Anhydride formation wasdetected by monitoring its characteristic stretching band in thecarbonyl region at 1738 cm⁻¹ [14].

Swelling Ratio

Equilibrium swelling measurements for all the samples were carried outin distilled water using a Sartorius microbalance (10⁻⁵ sensitivity).The swelling ratio was measured by weighing samples before and aftertheir immersion in distilled water for about 24 h. The swelling ratio(SR) is defined as following:

SR=(W _(s) −W _(d))/W _(d)

where W_(s) is the weight of the swollen polymer hydrogel and W_(d) isthe weight of the dried sample [15].

Differential Scanning Calorimeter

A differential scanning calorimeter (Mettler-Toledo 822^(e) Mettler DSC)was used for thermal analysis. The scanning temperature range and theheating rate were 10-200° C. and 5° C./min, respectively.

The adopted thermal cycle was: (1) heating 10-100° C.; (2) isotherm at100° C. for 3 minutes; (3) cooling from 100° C. to 10° C.; (4) heatingfrom 10° C. to 200° C.; (5) isotherm at 200° C.; (6) cooling until roomtemperature. An empty pan was used as a reference.

Fourier Transformed Infrared Spectroscopy

All FT IR spectra were recorded on a JASCO FT IR 660 plus spectrometerequipped with an attenuated total reflectance (ATR) crystal sampler.Film samples were used directly on a ATR crystal sampler at a resolutionof 4 cm⁻¹, by 300 scans, at absorbance range from 4000 cm⁻¹ to 600 cm⁻¹.

Results and Discussion

A DSC thermogram of neat citric acid showed a peak at about 60° C.,attributable to a water loss process associated with the dehydrationleading to an anhydride. A complete degradation, starting at about 160°C., is observed in the second scan.

DSC analysis of neat CMCNa and HEC powders indicates that some water isstill absorbed in the polymers. Above 100° C. a possible degradationpeak of CMCNa is detected. Both CMCNa and HEC show a thermal stabilitybelow 100° C.

A film of polymer hydrogel obtained using a 3:1 ratio of CMCNa/HEC and3.75% by weight of polymer of citric acid was analyzed by DSC afterdrying the sample at 30° C. for 24 h and then reduced to powder. A largeendothermic peak associated to the evaporation of the water produced bythe anhydrification process is evident. A small exothermic peak,attributed to esterification is superimposed on the first one. In thesecond heating cycle the glass transition (T_(g)=38° C.) of thecross-linked cellulose mixture is observed.

After this preliminary DSC study, different polymer hydrogel sampleswere prepared according the following procedures. After mixing reagentsin water, the reaction vessel was kept at 30° C. for 24 h in dryconditions to remove water. Then temperature was raised above 60° C.,according with the results of the first DSC analysis, in order to obtainthe citric acid anhydride. Above this limiting temperature citricanhydride is available for the cross-linking reaction with cellulose OHgroups. Different reaction conditions were attempted in order tooptimize the synthetic procedure, such as temperature and CAconcentration as summarized in Table 1. Two different reactiontemperatures for the cross-linking process, 80° C. and 120° C., wereattempted. However, either to prevent degradation risks or to limit thereaction rate a temperature of 80° C. was chosen. Moreover, very highconcentrations (10% and 20% by weight) of CA were initially used inorder to amplify the FT IR signals associated with each chemicalreaction step. First neat CMCNa and HEC were cross-linked with CA inorder to investigate its reactivity with each of the polymers.

TABLE 1 Reaction Initial citric acid concentration label polymer (% w/wpolymer) A10 CMCNa 10 A20 CMCNa 20 B10 HEC 10 B20 HEC 20 C10 CMCNa/HEC(3/1) 10 C20 CMCNa/HEC (3/1) 20

FT IR spectra were recorded of citric acid, of the A10 reaction mixturebefore heating and of the A10 reaction mixture after 5 h of heating. Inthe CA spectrum it is possible to observe a strong C═O band centred at1715 cm⁻¹ due to carboxylic acid. The FT IR spectrum of sample A10 showsa strong absorption band at 1590 cm⁻¹ characteristic of cellulose [16].After heating, the absorbance band at about 1590 cm⁻¹ is still observedand additionally a new band at 1738 cm⁻¹ appears. Anhydrides display twostretching bands in the carbonyl region around 1758 cm⁻¹ and 1828 cm⁻¹.The higher frequency band is the more intense in acyclic anhydrideswhereas cyclic anhydrides show the lower frequency C═O stretching bandstronger than the stretching band at higher frequency [14]. The new peakobserved at 1738 cm⁻¹ can be attributed to the characteristic stretchingband of the carbonyl group at lower frequency related to anhydrideformation, an intermediate reaction necessary for reaction of CA withcellulose hydroxyl groups. In contrast, the carbonyl peak expected athigher frequency is not detectable probably due to its weak intensity.

FT IR spectra were recorded of citric acid, B10 reaction mixture beforeheating and B10 reaction mixture after 6.5 h of heating. The HECspectrum again shows the band at 1590 cm⁻¹ before and after heatingwhile the absorbance of the carbonyl group at 1738 cm⁻¹ appears onlyafter heating at 80° C. as observed for the sample A10.

Although FT IR analysis is generally considered a qualitative technique,a literature study in literature carried out by Coma and co-workersdemonstrated that infrared spectroscopy could be used at firstapproximation for the determination of the cross-linking rate incross-linked cellulosic derivatives [9]. Starting from thisconsideration, the evolution of the different reactions finally leadingto cross-linking at 80° C. was monitored by recording FT IR spectra atdifferent reaction times.

The area under the absorbance peak at 1738 cm⁻¹ (A₁) representative ofthe carbonyl group, was compared with the area under the referenceabsorbance peak at 1592 cm⁻¹ (A₂) which is invariant in all spectra. Theevolution of the anhydride was evaluated as the ratio of A₁/A₂ as afunction of the reaction time. FTIR spectra of CMCNa polymer when thereaction is performed at 80° C. with 20% CA or 10% CA both show asimilar trend: the anhydride band, that is absent before heating,reaches a maximum almost immediately after the first hour, successivelydecreases to a minimum after 3 h, then increasing again reaching asecond maximum after 5 h. Finally a slower process reduces the band areato zero after 24 h. It is worth noting that in the spectrum of the 20%CA reaction, the second maximum matches a value (A₁/A₂=0.10) higher thanthose observed in the 10% CA reaction (A₁/A₂=0.04).

It is assumed that the peak at around 1738 cm⁻¹ is due to theanhydrification process involving free CA followed by the firstcondensation of this anhydride with cellulose OH leading to a fastdisappearance of the anhydride C═O groups. Then the carboxylate groupsnow linked to the polymer are able to form again an anhydride leading toan increase of the 1738 cm⁻¹ peak. The second reaction of this anhydrideis responsible for the cross-linking, results in a new anhydride groupconsumption and consequent reduction of the peak at 1738 cm⁻¹. Thissecond reaction is slower since it involves groups linked to largemacromolecules and hence is more sterically hindered, as has also beenreported for other cellulose cross-linking processes [17]. This possiblereaction mechanism is confirmed by the swelling measurements.

FTIR spectra were also recorded for reactions of HEC polymer when thereaction is performed at 80° C. with either 20% CA or 10% CA. In the 10%CA case, the anhydride band intensity increases from 0 to 0.098 when thereaction time increase from 0 h to 6.5 h, but drops to 0 when thereaction time reaches 24 h. The 20% CA reaction follows exactly the sametrend providing a maximum value of 0.079 at 5 h. Assuming that thecross-linking mechanism is the same as described for CMCNa, theanhydrification and esterification reactions appear superimposed in thiscase. Therefore, in the FTIR spectra, the HEC polymer shows a singlepeak. This latter result was in accordance with conclusion of Xie andco-workers [18]. They studied the degree of substitution, as evaluationof cross-linking esterification, on starch thermally reacted with CA atdifferent reaction time and found a maximum after a few hours.

To explain the data observed in all FTIR spectra recorded after 24hours, we supposed that in all cases the polymer is unstable when iskept in the oven for 24 hours because of unidentified secondaryreactions take place modifying polymer structure that involve also esterfunctions. Xie and co-workers [18] work guessed that the degree ofsubstitution reached a maximum and then decreased since dissociation ofthe substituents from starch occurred when the reaction time was longerthan 7 h.

Finally, to complete this study, polymer mixtures of CMCNa and HEC werecross-linked. CMCNa has carboxylic acid functional groups in itsstructure that increase the volume variation process in solution. Apreliminary attempt to follow the reaction pathway failed. Probably thereaction systems considered are too complex having many differentreaction centers. FT IR spectra of C10 reaction registered before, after8 h and after 13 h of heating were compared. Reaction sample C20 showssimilar spectra. Moreover, it is worth noting that when polymer mixturesare used (C10 and C20) a broad signal appears at about 1715 cm⁻¹,especially when a higher CA concentration is used in the reaction. Infact, with 20% of CA the signal of CA at 1715 cm⁻¹ is very broad andoverlapped to the polymer signal at 1590 cm⁻¹ then a clear band is notdetectable. However, it should be pointed out a band around 1715 cm⁻¹before heating. The C10 reaction mixture before heating shows a bandaround 1715 cm⁻¹ covering the absorbance region monitored previously forthe other reactions (A10, A20, B10, B20); consequently a clearassignment to the carbonyl group is difficult. However, the other twospectra indicate that this band moves to higher wavenumbers during thecross-linking reaction. In particular after 8 h, the FT IR spectrumshows a broad band in the range of 1711 cm⁻¹-1736 cm⁻¹ and after 13 hthis band appears more clearly as a narrow absorbance band at 1737 cm⁻¹,which is typical of carbonyl groups. Spectra of C20 reaction providesimilar results. Although a quantitative analysis of carbonyl groups isnot possible when C10 and C20 samples are cross-linked, an evaluation ofthe carbonyl peak similar to those observed for the reaction of the neatpolymers can be assumed.

The cross-linking kinetics were also monitored studying the swellingbehaviour during the reaction progress. Swelling ratio was calculated asa function of the reaction time for: (a) CMCNa with 10% or 20% of CAconcentration; (b) HEC with 10% or 20% of CA concentration; (c) themixture of CMCNa and HEC (3/1) with 10% or 20% CA concentration; (d) themixture of CMCNa and HEC (3/1) with 1.75%, 2.75% or 3.75% CAconcentration.

The results obtained indicate that the swelling of CMCNa cross-linkedwith 10% of citric acid is higher than HEC with the same citric acidconcentration after 24 h. When 20% of citric acid was added to thecelluloses, the shape of the swelling curves are similar for HEC andCMCNa. In this case, as cross-linking proceeds the swelling of HEC basedsamples decreases faster than CMCNa samples indicating that a higherrate of reaction between CA and HEC. This probably occurs because HEC isless sterically hindered than CMCNa and can react more quickly thanCMCNa chains. In addition in each repeating unit, HEC has more OH groupsthan CMCNa (3 vs 2).

The maximum swelling of CMCNa/CA sample is observed at the gelationonset, after 3 h, when the second esterification reaction, those leadingto cross-linking, begins. Then as the cross-linking process increasesthe corresponding equilibrium water sorption decreases, confirming theresults of FTIR analysis.

The same reaction mechanism can be assumed for neat HEC cross-linkedwith CA. However in this case the overall behaviour is slightlydifferent as a consequence of the absence of carboxylic groups bonded tothe polymer. The results of swelling experiments must be interpretedtaking into account that the CA introduces the high hydrophiliccarboxylic groups that are responsible of the formation of apolyelectrolyte network. Therefore the water sorption is significantlyincreased as carboxylic groups are linked first to the HEC chains andthen to the gelled network. This effect cannot be appreciated in CMCNapolymer hydrogels since a large amount of —COOH groups, those linked tothe CMCNa chains, is already bonded to the network at the onset ofgelation. A similar trend is observed for the mixtures of HEC and CMCNa.

Polymer hydrogels of practical use presenting a high degree of swellingwere obtained with a reduced concentration of citric acid (1.75%, 2.75%,3.75% by weight of polymer). With a citric acid concentration of 3.75%the swelling ratio can reach 900. This polymer hydrogel, after swelling,is characterized by adequate stiffness and it is able to keep the sameshape of the synthesis vat. Polymer hydrogels formerly synthesized [13]using divinyl sulfone, a toxic reagent, as cross-linking agents and thesame ratio between CMCNa and HEC were characterized by a maximumswelling ratio of 200. In this case a higher swelling ratio is obtainedusing an environmentally friendly cross-linking agent. At concentrationslower than 1.75% CA, a weak cross-linking associated with insufficientmechanical property is observed.

Conclusions

This work shows for the first time that CA can be successfully used ascross-linking agent of CMCNa/HEC mixtures. As shown in FIG. 1, anesterification mechanism based on an anhydride intermediate formation isproposed to explain the reaction of cellulose polymers with CA.

The cross-linking reaction for CMCNa/HEC system was observed either byDSC or by FTIR analysis. The evolution of the different cross-linkingreactions was monitored by means of FT IR spectra collected at differentreaction times using an excess of citric acid. The swelling ratio,monitored at different reaction times, confirmed the reaction pathfigured out from FTIR analysis. An optimal degree of swelling (900) forpractical applications was achieved using low CA concentrations. Thepolymer hydrogel obtained through the method described in this Example 1has the great advantage to reduce primary and production costs and avoidany toxic intermediate during its synthetic process.

Example 2 Citric Acid Cross-Linking of Carboxymethylcellulose andCarboxymethylcellulose/Hydroxyethylcellulose Mixtures in the Presence ofSorbitol Materials and Methods

All the materials employed were provided by Aldrich Italia and were usedwithout any further modification. The devices used in thecharacterisation, in addition to the standard laboratory glassware,cupboards and counters for standard synthesis, were a scanning electronmicroscope (SEM) JEOL JSM-6500F, a precision 10⁻⁵ g Sartorius scale, anIsco mixer and an ARES rheometer.

The polymer hydrogels were prepared by cross-linking an aqueous solutionof carboxymethylcellulose sodium salt (CMCNa) and hydroxyethylcellulose(HEC), using citric acid (CA) as the cross-linking agent and sorbitol asthe molecular spacer. The composition of a gel is given by the nominalamount of the reagents in the starting solution. The parameters used todefine said composition are the following:

the precursor weight concentration (%)=the total mass of polymers in thesolution (e.g. CMCNa+HEC) (g)×100/mass of water (g);  (i)

the CMCNa to HEC weight ratio=mass of CMCNa (g) in the solution/mass ofHEC in the solution (g);  (ii)

the cross-linking agent (CA) weight concentration (%)=mass of CA in thesolution (g)×100/mass of the precursors in the solution (g); and  (iii)

the molecular spacer (e.g. sorbitol) weight concentration (%)=mass ofmolecular spacer (g)×100/mass of water (g).  (iv)

The laboratory tests demonstrated that a polymer concentration lowerthan 2% and a CA concentration lower than 1% either do not achievecross-linking of the gel or led to the synthesis of a gel having verypoor mechanical properties. On the other hand, CA n concentrationshigher than about 5% significantly increase the cross-linking degree andpolymer stabilization, but excessively reduce the absorption propertiesof the superabsorbent gel.

Since CMCNa is the ionic polymer species, it is possible to achieve thedesired absorption properties adjusting the weight ratio ofcarboxymethylcellulose sodium salt (CMCNa) to hydroxyethylcellulose(HEC). A CMCNa/HEC weight ratio of between 0/1 and 5/1, preferablybetween 1/1 and 3/1, was observed to enable the synthesis of a polymerhydrogel having optimum absorption properties.

Examples relating to the synthesis of different polymer hydrogelsaccording to the invention, differing from one another in the weightpercent (wt %) of citric acid and in the composition of the polymericprecursor, are provided below.

Preparation of polymer hydrogel A: in a beaker containing distilledwater, sorbitol at a concentration of 4% by weight referred to theweight of distilled water was added and mixed until completesolubilisation, which occurred within a few minutes. The CMCNa and HECpolymers are added at a total concentration of 2% by weight referred tothe weight of distilled water, with a CMCNa/HEC weight ratio of 3/1.Mixing proceeded until solubilisation of the whole quantity of polymeris achieved and the solution became clear. At this stage, citric acid ata concentration of 1% by weight referred to the weight of the precursorwas added to the solution, whose viscosity had greatly increased. Thesolution thereby obtained was poured into a vessel and dried at 48° C.for 48 hours. During this process, the macromolecules are stabilisedinto a polymeric network which is the backbone of the polymer hydrogel.At the end of the cross-linking process, the polymer hydrogel was washedwith distilled water for 24 hours at room temperature. During thisphase, the polymer hydrogel swelled up thereby eliminating theimpurities. In order to obtain the maximum swelling degree andelimination of all of the impurities, at least 3 rinses with distilledwater were performed during the 24 hours washing step. At the end ofthis washing step, the polymer hydrogel was dried by phase inversion inacetone as the nonsolvent, until a glassy white precipitate is obtained.The precipitate is then placed into an oven at 45° C. for about 3 hours,to remove any residual trace of acetone.

Preparation of polymer hydrogel B: Polymer hydrogel B was prepared aspolymer hydrogel A, with the only exception that the polymer is madeonly of CMCNa, and that the CMCNa concentration is 2% by weight referredto the weight of distilled water.

Preparation of polymer hydrogel C: Polymer hydrogel C was prepared aspolymer hydrogel B, with the only exception that the citric acidconcentration is 2% by weight referred to the weight of CMCNa.

Preparation of polymer hydrogel D: Polymer hydrogel D was prepared aspolymer hydrogel B, with the only exception that the citric acidconcentration is 0.5% by weight referred to the weight of CMCNa.

Absorption Measurements

In order to test the absorption properties of the polymer hydrogelsprepared as described above, they were subjected to absorptionmeasurements in distilled water. The absorption measurements essentiallyconsist of placing the dry sample, obtained from the drying step, indistilled water, so that it swells up until an equilibrium condition isreached.

The absorption properties of the gel are assessed based on its swellingratio (SR), defined according to the formula illustrated above. In orderto minimise the influence of experimental errors, each test wasperformed on three samples from each gel, and then the mean value of theresults of the three measurements was taken as the effective value.

Three dry samples were taken from each of the test gels, each havingdifferent weights and sizes. After recording the weights, the sampleswere swollen in abundant quantities of distilled water at roomtemperature. Upon reaching equilibrium after 24 hours, the samples wereweighed once more in order to determine the swelling ratio.

Results

Table 2 below reports some of the results obtained, in terms of theswelling ratio, varying the concentrations of the reagents and thecross-linking times (6 hours, 13 hours, 18 hours, 24 hours).

TABLE 2 Sample CMCNa HEC CA sorbitol cross-linking time/swelling ratio —75% 25% — — 6 hours 13 hours 18 hours 24 hours g16 2% 1% 4% nr 50 30 20g17 4% 1% 4% nr 25 10 5 nr = not cross-linked

It is pointed out that the increase in the polymer concentration exertsa negative effect on the swelling properties of the final product and itis also pointed out that the cross-linking time exerts a significanteffect of the absorbing properties.

Thus, further experiments were carried out maintaining the polymerconcentration fixed at 2% and varying the citric acid concentration. Theresults are reported in Table 3.

TABLE 3 Sample CMCNa HEC CA sorbitol cross-linking time/swelling ratio —75% 25% — — 6 hours 13 hours 18 hours 24 hours g21 2% 2% 4% 40 25 20 10g22 2% 1% 4% nr 50 30 20 g23 2% 0.5%  4% nr Nr 50 30 nr = notcross-linked

Table 3 shows that the sample having the best swelling ratio is thesample designated as g22, which is characterised by a citric acid (CA)concentration of 1%.

Thus, further experiments were performed removing completely HEC fromthe solution. This should render the polymer hydrogel more hydrophilicthereby leading to an increase of the swelling ratio. Table 4 shows someof the results obtained.

TABLE 4 CMCNa HEC CA sorbitol cross-linking time/swelling ratio Sample100% 0% — — 6 hours 13 hours 18 hours 24 hours g30 2% 2% 4% nr 85 55 30g31 2% 1% 4% nr 100 75 40 g32 2% 0.5%  4% nr Nr 70 50 nr = notcross-linkedThe highest swelling ratio is associated with a cross-linking time of 13hours and a citric acid concentration of 1%. It is also to be noticedthat higher citric acid concentrations together with shortercross-linking times lead to equally satisfactory swelling ratios,although the reaction is very fast and less easy to control.

Finally, the possibility of increasing the swelling ratio by creatingporosity into the material which could promote the absorbing properties,was evaluated. For that purpose, the sample g31, subjected tocross-linking for 12 hours, was swelled into distilled water for 24hours and then dried by phase inversion in acetone. With this technique,a swelling ratio of 200 was obtained.

Example 3 Swelling of a Polymer Hydrogel in Simulated Gastric Fluid(SGF) and SGF/Water Mixtures

This example describes an evaluation of the superabsorbent polymerhydrogel denoted polymer hydrogel B in Example 2 in in vitro swellingand collapsing experiments in various media at 37° C.

Swelling Kinetics (in 100% SGF) at 37° C.

100 mg of the dried polymer hydrogel was immersed in either simulatedgastric fluid (“SGF”) or a mixture of SGF and water and allowed to swelluntil an equilibrium condition was reached. SGF was prepared accordingto USP Test Solutions procedures. The swelling ratio in each fluid wasdetermined at various time points. The results are set forth in Tables 5and 6.

TABLE 5 Swelling of dry polymer hydrogel B in 100% SGF at 37° C. Theweights were recorded at 15, 30, 60 and 90 min. Swelling Swelling Time,min Ratio, g/g 15 15.4 30 15.6 60 16.2 90 15.1

TABLE 6 Swelling of Dry Polymer hydrogel B in a mixture of SGF and Water(1:8) at 37° C. The weights were recorded at 15, 30, 60 and 90 min.Swelling Swelling Time, min Ratio, g/g 15 78.8 30 84.6 60 88.6 90 79.3Collapsing Kinetics (with Addition of SGF) at 37° C.

To simulate the effect of digestion on a hydrated polymer hydrogel, tothe swollen polymer hydrogel from above (Table 6, SGF/water) after 60minutes, 100% SGF was slowly added to collapse the gel particles.Swelling ratio was monitored as a function of cumulative volume of addedSGF. The results are set forth in Table 7.

TABLE 7 SGF Swelling added (mL) Ratio (g/g) 0 88.6 8 23.1 30 22.6 5023.1 75 17.1

Kinetics of Swelling (in 1:8 SGF/Water), Collapsing (in SGF) andRe-Swelling (in Simulated Intestinal Fluid)

Experiments were conducted by monitoring the swelling ratio through afull cycle of swelling in 1:8 SGF/water, collapsing in SGF, andre-swelling (then degradation) in simulated intestinal fluid (SIF), allat 37° C. Experiments performed and results are provided in Table 8, forthe re-swelling/degradation kinetics. pH values are given whenavailable.

TABLE 8 Kinetics of swelling in SGF/water, collapsing in SGF, andre-swelling in SIF 60-min Swell in Collapse in Expt. SGF/water 70-mL SGFRe-swelling/Degradation in SIF # Swell Ratio Swell Ratio 30 min 45 min90 min 120 min 1 95.5 20.7 71.2 87.3 pH 4.82 pH 1.76 2 95.3 19.5 72.680.5 pH 1.75

Conclusions

This polymer hydrogel swells in simulated gastric fluids (pH 1.5)approximately 15 fold, and in a simulated gastric fluids/water mixture(pH 3) approximately 85 fold. This indicates that the polymer hydrogelhas a pH/swelling correlation where at pH below 3 (pKa of CMC is ˜3.1)there will be limited swelling of the polymer hydrogel due to absence ofthe Donnan effect. The polymer can also swell in the increased pH ofsimulated intestinal fluid.

Example 4 Effect of Polymer Hydrogel on Rat Feeding Behavior

A series of experiments was conducted to assess the effect of polymerhydrogel B in laboratory animals. One objective of these studies was todetermine the effect of polymer hydrogel B on food intake in rats. Thestudy was conducted in male Sprague Dawley rats, by acute administrationof pre-swollen polymer hydrogel B by oral gavage.

A total of 22 male Sprague-Dawley rats were randomized into twoweight-matched groups prior to polymer hydrogel or vehicleadministration (the polymer hydrogel B was pre-swollen in water, 100 mgin 10 mL water). Food and water intake (digital balance) as well aslocomotor activity (consecutive beam brakes) were monitored online every5 minutes for 40 hours post dosing. Food and water intake data werecollected using MaNi FeedWin, an online computerized feeding systemusing digital weighing cells. Two types of baseline food intake (digitalbalance) and lick counts were monitored. All data was entered into Excelspread-sheets and subsequently subjected to relevant statisticalanalyses Results are presented as mean±SEM unless otherwise stated.Statistical evaluation of the data is carried out using one-way ortwo-way analysis of variance (ANOVA).

Results and Conclusions

FIG. 2, a graph of cumulative food intake as a function of time,represents a typical study result. There was no difference between thegroups at base line. Gavage of 8 mL of polymer hydrogel B inducedsatiety in the rats that led to a significant decrease in food intake.As shown in the yellow line, this polymer hydrogel induced a markeddecrease in food intake that persisted over 2 hours. These data suggestthat polymer hydrogel B can induce satiety in animals and leads to adecrease in food intake.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

REFERENCES

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1. A method for preparing a polymer hydrogel, comprising the steps of:(a) providing an aqueous solution comprising a hydrophilic polymer and apolycarboxylic acid or an anhydride thereof, wherein said polycarboxylicacid is a C₄-C₁₂-dicarboxylic acid, a tricarboxylic acid or atetracarboxylic acid; and (b) maintaining the solution of step (a) underconditions suitable for cross-linking of the hydrophilic polymer by thepolycarboxylic acid; thereby forming a polymer hydrogel. 2-51.(canceled)